Vibrating condenser



3 Sheets-Sheet l .27m/envan',

Nov. 18, 1930. H. VOG-'r VIER/mm;A coNDENsER Filed Sept. 6, 1927 ssheetssheet 2 Nov. 18, 1930. l H. VOGT VIBRATING CONDENSER Filed Sept.6. 1927 5 Sheets-Sheet 5 Fly Z.

Patented Nov. i 18, 1930 UNITED STATES PATENT oFFlcE- `HANS VOGT, OFBERLIN-WILMERSDORF, GERMANY 'VIBRATING CONDENSER Application ledSeptember 6, 1927, Serial No. 217,838, and in Germany April 28, 1926.

invention relates to the production of t sound waves in the air by wayof electricity particularly for purposes of receiving and reproducingsounds or tones of the human voice I and other natural sounds, music,tunes and anyother tones or sounds perceptible to the human ear.

An object of my invention is to provide an apparatus or device for theproduction and reproduction of sound waves or sounds of the stated kindthe intensity of which can be controlled at Will. Another object of myinvention resides in the particular construction and coniguration ofsingle parts or members of the said apparatus andthe assemblage andinterconnection thereof to constitute the apparatus as a unit. A chieffeature of my invention, moreover, resides in the particularconstruction and arrange-` ment of"a metallic diaphragm adapted to beattached to a suitable frame or apparatus such as a telephone, aloud-speaker for radio reception, a grand or full-volume loud-speak- 4er and the like, and to be caused to vibrate,

to impart its vibrations to the air for the production or reproductionof sounds of any desired intensity within a controllable range.

Apparatus of the kind as hitherto con- 39 structed and which areequipped with a diaphragm adapted to vibrate or oscillate in accordancewith the electrostatic principle for the purpose of convertingelectricity to sound are objectionable for insucient eiiciency and, asfar as my knowledge goes, this disadvantage is mainly due to theemployment of unsuitable diaphragms, since the diaphragm is the mostimportant member or constituent of an apparatus of this type.

Diaphragms as hitherto suggested for the purpose mainly consists oforganic matter such as rubber, paper, Celluloid, gold beaters skin,leather and the like overlaid With'a conductive coating. Diaphragms ofthis kind,

however,`are objectionable for the reason that- 5 ther that the oxygenof the air exerts a detrif" according to the electrostatic principle, soasv mental action thereon so as to destroy or reduce the desirable goodproperties thereof.

For this reason, it is true, it has been suggested heretofore to usediaphragme made of a heavy metal such as iron or'steel, but the 55 highspecific gravity of such metals has 'proved incompatible with theacoustic purposes of a diaphragm, the more s'o since it is impossible toreduce the thickness of an iron sheet, considering the comparativelygreat diameter of a diaphragm of up to 50 cm. and still more, to between10 and 70, thousandths of a millimeter as required for transmitting thelowest sound frequencies at or with a relatively large amplitude. 6"

Furthermore it has been proposed heretofore to employ thin mica discs orfilms, but although they will give better results, still they are 'notto be had in the trade and the high manufacturing cost and the fragilitythereof are also a bar to any extensive use of the same. Besides thatmica is, just as paper, rubber, celluloid and the like, objectionablefor the so-called dielectrical residual or remanentcharge effects, suchresidual charges remaining in the material and acting to disturb, in anentirely uncontrollable manner, the electric potential ortensionconditions and consequently the vibratability of the diaphragm.Moreover, it was impossible heretofore to give a comparatively largefreely vibrating diaphragm the required mechanical tension and tosoemount the same with relation to the. stationary, immovable grid-plateto be charged electrostatically, that the dia- 35 phragm will' becapable of vibrating bodily or asa whole Without' performing naturalvibrations or vibrations of its own, so as to obtain a clearreproduction of the deep or low tones.

For the purpose of preventing the resonances that is to say theoccurrence of the said natural vibrations or self-vibrations of thediaphragm acting toyfalsify the reproduced sound or tone, -it has beensuggested heretofore to subdivide the diaphragm into a plurality ofeccentric rings or sections or alternatively to provide means forsupporting the/diaphragm at the centre thereof or at certain pointsbetween the usual grid-plate and the diaphragm. lin this way, it istrue, the fundamental oscillations of the diaphragm will be reduced, butirregular upper or harmonic oscillations frequently set up which imparta spluttering nasal or crackling character to the sounds reproduced byan electrostatic loud-speaker, and the efficiency thereof is greatlyreduced due to the several supported' points which will not partake inthe oscillations or vibrations of the diaphragm.

In order to balance this reduction in or loss of efficiency to a certaindegree, it has been suggested to use considerably high potentials orvoltages such as 500 to TO0 volts for operating the loud-speaker whichobviously necessitates or involves correspondingly great variations involtage. For this reason vibrating condensers for use as loudspeakersfor broadcasting or radio telephone reception did not meet with successand did not come into fashion, the less so as, according to the presentstate of the art, it'is impossible to accurately control and adjust therequisite gap or spacing between the diaphragm and the grid-plate overthe whole of the surfaces thereof in order to be enabled to correctlyadjust the parts to conform to the voltage and the variations thereoffor the time being without thereby impairing the acoustic properties andreducing the electro-acoustic efficiency of the same.

According to my invention the problem of a vibrating or oscillatingcondenser adapted for use as a loud-speaker, headphone or microphone issolved and practically accomplished in a most perfect and efficientmanner by employingr for the purpose in view an extremely light andextremely elastic metallic diaphragm of high tensile strength and whichis given, in case of a loud-speaker, a large size exceeding far beyondthe dimensions customary in diaphragms of the kind. My invention furthercomprises means for periodically imparting to the said diaphragm.,independently of outer influences, ample vibrations devoid of resonanceat relatively low voltages and for adjusting the spacing or gap betweenthe diaphragm and the grid-plate so as to thereby realize strictconformity with the prevailing voltage.

Other important features of my invention will be hereinafterspecifically describedand then pointed out in the claims. The inventiveidea involved obviously is capable of expression in a variety ofmechanical forms one of which, for the purpose of illustration, issho-wn in the accompanying drawings, but it is to be understood thatsaid drawings are for the purpose of illustration only, and not for thepurpose of defining the limits of the invention, reference being had tothe claims for the purpose.

In the drawings forming a part of this specication I have shown avibrating/conmeagre denser for use as a loudspeaker and a modifiedconstruction thereof for use as a telephone or microphone, bothembodying my invention, but l wish to have it understood that the shownconstructions are not the only embodiments of which the invention iscapable, that a wide range of equivalents may be employed for partswhich l have illustrated and that the phraseology which l have adoptedis for the purpose of description and.

not of limitation except where the sense absolutely requires it.

ln the drawings:

Figure l is a perspective view of a vibrating or sounding'condenserconstructed in accordance with the present invention;

Figure 2 is `a section taken on the Vline lli-lll in Figure l at alarger scale;

Figure 3 is a front view of the grid-plate of the condenser Figure i isa similar view;

Figure 5 is a fragmentary view similar to Figure l with a base body orpedestal shown in section;

Figure 6 is a sectional view of the circumferential parts or clampingrings of the condenser shown on a larger scale to clearly illustrate themanner in which the grid-plate and the diaphragm are clamped between therings; I

Figure 6 is a sectional view showing the details of Figs. 6, 9 and 10combined into a single form of the invention;

Figure 7 is a section taken on the line Vil-VH of Figure (i:

Figure 8 is a vertical section showing the adjusting device for settingor controlling the sound intensity;

Figures 9 and 10 are sectional views of an auxiliary device fortensioning the diaphragm with the tensioning elements shown in twodifferent positions;

Figure 1l is a front view of the condenser in a sounding box or casingshown on a reduced scale;

Figures 12a and 12b are a sectional view and a front view of a similarapparatus; and

Figure 13 is asectional view of the modification for use as a telephoneor microphone.

Parts which are repeated in the several figures hear the same referencecharacters in each case.

2() denotes the metallic diaphragm. Best results, as regards a loudreproduction free from distortions, are obtained by manufacturing thediaphragm of a material which is light in weight and as hard as it canbe. Respecting density or tensional strength steel obviously would bethe very best material. Up to now, however, it has been impossible toroll steel down to a minimum thickness of 5 thousandths of a millimeter,so 4that steel cannot be employed with a view to ensure satisfactoryresults." Further light metals such as aluminium cannot be used,jbecause thinness, say of 5 to 10 thousandths of a millimeter, with atensile strength of 32 to kg. per square millimeter, will give bestresults. As regards the refining operation I prefer to refine the alloysubsequently to the l, rolling operation by heating and suddenly coolingthe sheets or films preferably while,` they are in a stretched ortensioned condition in order to remove all unevenness and irregularitiesin 'the material.

The vibratory mass of the diaphragm is to be kept as low as possible inconsideration of the reproduction of the high tones and the necessity ofpreventing natural vibrations. In researches made to the end I foundthat electrostatic loud-speakers operated withV the aid of voltages of100 to 300 volts and consequently possessing a certain electrostaticfield of limited strength, are not capable of uniformly reproducing ortransmitting the practical acoustic range of to 12000 Hertz eX- cept ifthe vibrating mass thereof does not exceed beyond a certain value. Asthe vibratory mass of a diaphragm depends upon or is defined, on the onehand, by the specific gravity of the material and, on the other hand, bythe thickness thereof, both values are to be considered and the productresulting therefrom-With the thickness indicated in millimeters-is to beinserted into the calculation. I have found that thefigure in- If it isdesired to come to or obtain the same value in connection with adiaphragm made of steel, the latter ought to be of less thickness,proportionate to the difference in the specific gravity, that is athickness of 13 i thousandths of a millimeter- I prefer to use alloysmanufactured according to the modern development of the art,particularly an aluminium alloy containing magnesium or zinc and endowedwith the properties of hightensile strength and unlimited rollingcapability. The aluminium alloy universally known as duralumin andcontaining manganese and copper is of no avail for the reason that itcannot be rolled to the required degree of thinness. On the other handan aluminium alloy such as magnalium consisting of pure aluminium (up to99.5 percent) and magnesium (up to 3.5 percent) answers. the purpose ina very satisfactory way. rIhe thickness of the diaphragm may `varybetween 5 and 80 thousandths of a millimeter.

Referring to the drawings, 21 denotes the grid-plate made of asuitablemetal or alloy by way of a casting operation or inv any other suitablemanner. I wish it to be understood, however, that also a grid-plate maybe used, in 'accordance With my invention, which consists of ,anyappropriate electrically insulating material capable of being cast ormolded such as ebonite, vulcanite, porcelain, artificial stone and thelike, but it goes without mentioning that in this eventuality thegrid-plate must be given an electrically conduct-ing coating at least onthe side or surface adjacent to the diaphragm. The gridplate 21 isprovided with a greatest possible lbrating diaphragm 20 and the adjacentlayer of air and reducing the air damping in the gap between thediaphragm and the gridplate. As it would be very expensive to producethe perforations by means of a stamping, boring or cutting machine andfor other reasons, I prefer to manufacture the plate inclusive of theperforations or apertures therein, according to the present invention,

by a. casting or pressing operation and a subsequent turning or grindingopera-tion for smoothing the plain surface of the plate. The saidcasting or pressing operation involves the necessity of .shaping theskeleton pieces or strips 211 bordering or confining Vthe saidperforations or apertures 210 in the form of a wedge so as to facilitatethe removal of the casting from the mold. l,

When assembled to form a condenser as shown in Figure 1 the diaphragm 20and the grid-plate 21 are mounted in parallelism and 1n close proximityto each other, as will beclearly seen in Figure 2. Prior `to suchassemblage the diaphragm 20 is to be subjected to a tensioning processby means of a suitable stretching device in order to impart to the samethe required highest possible radial tension. t

A device suitable for the purpose is shown in the Figures 9 and 10. Thesame comprises a strong metallic ring 22 with threaded bores for thereception of screws 25. The diaphragm 20 .provided with correspondingmarginal holes 23 is placed upon the ring 22 so as to'have the holes `23register with the..l o

bores of the ring 22z whereupon a second llf) metallic ring 24 of thesame diameter and provided with corresponding bores is placed upon thediaphragm so that all of the bores and holes will be in axial alinementfor the reception of the screws 25 which upon having been inserted andscrewed up, as shown in Figures 9 and 10, will hold the diaphragm firmlyenclosed between the jaw-like acting rings 22 and 24. The ring 22 isprovided with an annular groove or recess 28 on its inner face and thering 24 is provided with threaded bores for the reception oftensioningscrews 26 located so as to register with the said groove 2S,the inner ends or points of the screws 26 engaging in a circular grooveprovided on the inner face of the ring 24 and in which a metallic ring27 is encased. By screwing up the screws 26 the ring 27 will be forcedtowards and against the diaphragm and by penetrating more and more intothe annular groove 28, the ring 27 will cause the diaphragm 2O togradually recede into the annular groove 28 whereby the diaphragm.

will be tensioned gradually and in proportion as the screws 26 aretightened up. The tightening of the screws 26 is continued until almostthe limit of the tensile strength or resistan'ce of the diaphragm isattained.

' A ring 29 connected with the grid-plate 21 and the outer diameter of`which is smaller than the inner diameterl of the compound ring 22 and24, is provided or coated with a perforated insulating ring 30 and theten-.

sioning device with the diaphragm 20 therein is placed'upon and over thegrid-plate 21 and the outer ring 29 connected therewith, so

' that the peripheral portion of the diaphragm the perforations of thediaphragm and the ring 30 and the parts are assembled in a manner thatall of the perforations thereof accurately coincide or register.

The thus assembled parts are sandwiched, as a unit, between two clampingrings 32 and 35 as will be readily understood on inspection of Figure 6.The rings 32 and 35 are each provided or integral with perforatedprotective discs 36 and 37, respectively, and they are further providedwith holes or perforations exactly corresponding to the perforations ofthe several parts enclosed bet-Ween the rings 32 and 35 and discs 36 and37, an insulating sheet of oil-paper 390 having been previously inserted-between the ring 29 and the clamping ring 35 for the purpose ofinsulation and prevention forations 290 of ring 29. The bolts 33 arescrewed up into threaded bores of the counter-ring 35.

As shown in Figure 6 the bolts 33 may be of tubular form with internalthread for engagement with screws 34 fitting in bores of thecounter-ring 35 and engaging in the bolts 33 projecting into theelongated perforations 290 so that the assemblage and rigidinterconnection of the parts can be readily accomplished bv means of ascrew-driver as will be clearly understood on inspection of Figure 6. Inorder to prevent metallic contact between the screws 33 and theldiaphragm 2O a protective sleeve 47 of insulating`l material is fittedon each screw.

In order to remove the tensioning device from the structure thediaphragm may be cut out in the annular gap between the rings 3s, 35 andthe clamping rings 22, 24 of the tensioning device or the screws 26 and25 of the latter may be unscrewed so that the marginal portion of thediaphragm will freely project beyond the outer periphery' of the rings32 and 35 after the removal of the disengaged rings 22 and 24, and maybe trimmed ofi' as desired. At any rate the thus assembled andinterconnected parts constitute a structure which may be handled as aunit and subjected to any appropriate finishing treatment.

The protective-discs 36 and 37 provided with perforations-360 and 370,respectively, of any desired size. number and configuration, are coatedon the inside with a coloured textile fabric pervious to sound such assilktissue, liXed to the inner surfaces of the discs by means ofanysuitable agglutinant. A shallow' annular recess provided in the outerperipheral surface of the structure or rings 32 and 35 serves for theinsertion of a metallic ring or closed band 38 which will conceal theinturned edges of the diaphragm and the insulatingrings and form at thesame time an aesthetic closure. If it is desired to mount the structurein wooden plates, the band 38 may be provided with say three smallangle-pieces 380 each havingr one or several holes for screws or thelike.

The grid-plate 21 is not rigidly connected with the intermediate ring 29supporting the diaphragm 20, but a number of resilient radial steel pins39 are fitted in radial bores 40 of the ring 29 so as to engage with thein- The means for ensuring a rigid wardly projecting ends in holesprovided for 'the purpose in the rim 213 of the grid-plateV 21. Thesepins 39 prevent the somewhat elastic grid-plate 21 from getting unevendue to stresses setting up in the ring system 29, 32 and 35 during themounting operations. Furtherthe pins 39 will act to force' phragm sothat disturbances caused by wear-v relation to the ring 29 in order totherebyy minimize the useless capacity, and to this end I insertbushings or short sleeves of 1nsulating material into the radial holesprovided in the rim 213 of the plate 21 for the reception 4of the pins39. l

Byjoining the parts 32, 30, 20, 31, 29, 390 and by means of the screws33 and 34 to form the system or structure( described,

the capacity surface is bent out towards the rearside and away from thediaphragm owing to the elasticity of the pins 39. rIhe tension of thediaphragm thus produced, therefore, depends upon the resiliency of theyielding pins39. In order to afford the pins the possibility of yieldingunder the stress, the bores of the ring 29 are enlarged at their innerends and the enlargements 400,-

with similarly shaped intermediate solid webs, stays or strips 211forming a skeleton disc with the aid of interconnecting radial ribs 212on the rear face of the plate. rlifhe ribs 212 extend from the rim orperiphery 213 of the plate radially towards and to the hub 214 thereofand shorter reinforcing ribs 215 are provided between adjacent ribs212to ensure perfect rigidity of the segemental stays or strips 211having the largest diameter.

rIhe grid-plate 21 is constructed to allow of being bent out to theamount of a few thousandths of a millimeter. For the purpose ofadjusting the grid-plate to the correct distance, that is, spacingthesame with relation to the diaphragm, as required by and for a certainpotential or voltage, there is provided in the hub of the grid-plate anadjusting device for bending the latter out to the minimum amountrequiredfor the purpose, so.

that the plate and the diaphragm will -be 1n exact parallelismthroughout, that is to say, the width cf the gap between the centralZones of the plate and the diaphragm will be accurately the same as thatbetween the peri heral zones thereof, where the insulating rmg j Thesaid ad 41 determines the clearance. )listing device comprises aset-screw 42 engaging in a threaded bushing or nut 43 xed vin the hub ofthe grid-plate by means of a sleeve 430 of insulating material. Thesetscrew 42 is supported in a bearing 44 provided in the centre of thebulged out and consequently duly stayed protective disc 37., a spring 45coiled about the screw and the outer end of the bushing 43,l acting tonormally hold the disc 37 a certain distance apart or] spaced from thegrid-plate, so that the screw 42 cannot get loose. By properly settingthe screw 42 the grid-plate can be adl usted and spaced to assume oroccupy the best possible position vwith relation to the diaphragm forensuring a highest possible strength or loudness of sound at a given orpredetermined tension. This position lies a little before the unstablepoint of the diaphragm.

Inserted between the grid-plate and the diaphragm is a suitableinsulating layer in the form of an elastic sheet or film 46 which may beof collodion, silk-paper or the like. I prefer, however, to employinstead a sheet or film made of a material which possesses insulatingproperties and at the 'same time is electrically conductive in a slightdegree and I succeed in making sheets or films possessing the abovecharacteristics by mixing a suitable insulating material with asubstance or substances adapted and acting to reduce the insulatingproperty of the material. In this way I prevent the insulating sheet orfilm from retaining any residual or remanent charges since they willflow off into the diaphragm the very moment they set up. This is ofgreat importance as regards the desirability of obtaining aloud-speaking condenser of never failing efficiency. Furtherl aninsulating sheet or film of the stated kind.v

will prevent immediate contact between the diaphragm and the gridsplateand consequently short-circuiting, furthermore the same will prevent thevibrating diaphragm n from striking immediately and hard against thegrid-plate which otherwise, particularly if a hard insulating layer orsheet would be used, will impart to the sound a disagreeable splutteringnasal and whizzing timbre. Best results will be obtained by using asheet or film of utmost thinness not exceeding 15 thousandths of a`rnillimeter and free from in such a case since high frequency lossescannot happen on account of lack of any solid dielectric. Apparatus ofthe kind as hitherto suggested cannot be utilized for similarpurposesvfor the reason that, setting aside any other difficulties, theyare not constructed and arranged so as to stand the enormously highwattless high frequency current of about l0to 30 amperes.

The electrostatic condenser herein described may be connected with apedestal 48 by means of screws 49, as shown in Figure 5, and thepedestal preferably also serves for the accommodation of the chokingcoils, condensers, resistances and similar fittings/vrequired for thecompletion and operation of the apparatus. Y Two sleeves or sockets 50and 5l are provided in the front face ofthe pedestal for connect-ion bymeans of a twopin plug.

Sounds are emitted by the described loudspeaking condenserunrestrainedly from both sides thereof. In some cases particularly incase of a small apparatus wherein the dialneter of the efficientsurfaces does not exceed thirty centimeters, I provideV asoundstrengthening screen such as a casing or box made of wood,sheet-metal or the like, open on opposite sides, as shown in Figures 1l,12a and 12h.' The enclosure` surrounding the loud-speaker laterally andon top to form a broad reso-nance body 52 prevents slow soundfrequencies from being equalized too rapidly and causes them to taketheir way intothe open space. For this reason a loudspeaker of thedescribed type is particularly well adapted for use in constructionalconnection with broadcasting or radio appara tus, phonographs and,1 thelike. For surrounding wooden structures or casings which, however, mustbe` open at the rear, will assist in greatly improving the eiliciency ofthe loud-speaker as regards the upper or lower frequencies, and serve atthe same time for the accommodation of the required electrical parts orfittings 54 and 55, as shown in Figure 12a.

A diaphragm 20 of the described type but of correspondingly reduceddimensions and extreme thinness may just as well be employed inconnection With a headphone, which will be much superior to the usualmagnetic receivers, as regards understanding, since the same is free ofresonances and adapted to transmit or reproduce consonants of the orderof 4000 to 10000 Hertz in a more reliable manner than magnetictelephones. In constructional respects the dimensions of the singleparts constituting the apparatus must be correspondingly reduced andsimplified. f

In Figure 13 I have shown a structure, thus reducedV and simplifiedwhich is adapted for use as a headphone or as a microphone.

The diaphragm 20 is clampedbetween the aveaae front ring 56 and the rearplate or casing 5? by means of screws 58, rings of insulating materialbeing insertedV between the clamping and clamped faces. yIhregrid-plate2l is spaced a distance apartfrom the diaphragm by means of aninsulating ring 4l, and insulated against the casing 57 by means of aninsulating` ring -390 and an insulating sleeve.

or bushing 430 on the adjusting screw 59 abutting against an elasticplate 62 attached to the insulating ring 390 on the annular flange ofthe grid-plate. By screwing down the screw 59 the diaphragm 20 will begiven the required iinal tension. A wire 61 is provided for conductiveconnection with the grid-plate. A construction such vas shown in FigureI3 is highly sensitive and entirely independent of frequencies and canbe manufactured in a simple manner at comparatively low cost, so thatthe same will be highly adapted for use in telephony in lieu of thecustomary magnetic telephone, viewing the improved eiiiciencyparticularly as regards understanding.

lLoud-speakers of moderate and of maximum size, however, requirediaphragms of a diameter between 20 and 60 cm. in order that the lowtones reproduced thereby`V might be of sufficient loudness. If it isdesired to further increase the loudness and the quality of sound Isuggest to unite several loud-speakers of the described type in parallelconnection and to impart to the single loud-speakers different orvarying acoustic properties by varying the weights of the severaldiaphragms and by differently adjusting the spacing of the diaphragm andthe grid-plate in each individual apparatus.

As regards the arrangement of the diaphragm in the apparatus I againemphasize the necessity of tensioning the same to the utmost possibledegree within the ring 20 in order to obtainbest results which cannot berealized except when the distance or gap between the diaphragm and thegrid-plate is a minimum and perfectly uniform. Further the 'constructionand arrangement of the parts must be such that the same will beindependent of variati ons in temperature, that is to say, the normallyprevailing variations of temperature must not exercise any appre-'ciable' influence on the tension of the dia phragm.

According to the present invention I realize this requirement by makingboth the diaphragm 20 and the supporting ring 29 of metals possessingthe very same speciiic coeiiicient of expansion and by providing aresilient support. y V

vA further important feature of my invention resides in the provision ofmeans for compensating those diametrical differences or variations whichdepend `upon or are brought about by the different rate at which anyouter temperature is received and f" whole of the sound range.

sumed and which, therefore, depend upon the volume of the material to beheated. To this end I- provide on either side of the diaphragm astanding layer of air enclosed between the diaphragm and the protectivediscs 36 and 37, with the result that in case of fall of temperature thediaphragm cannot cool down and contract .more rapidly than thesupporting ring 29.

Furthermore, in reducing my invention to practice, it is of greatyimportance to subdivide or perforate in a suitable manner thegrid-platey 21 co-operating with the diaphragm 20. I have foundl thatthe perforations or openings of the gridfplate 21 must bear a certainratio in respect to the metallic skeleton thereof in order to reduce theair damping in the gap between thediaphragm and the plate and in thesaid perforations thereof to an adequate degree with relation to theemission of sounds and the suppression of the natural vibrations. It is,however, advisable to not go too far in this regard, that is to say, tonot provide more or larger perforations than absolutely necessary forthe purpose aimed at, since other- Wise the materialwould not possessany air damping and would be able to fully develop its natural`vibrations so as to disturb there'- bv the uniform reproduction of orover the By increasing or maintaining within adequate limits the totalarea of the metallic skeleton or strips defining and framing theperforations of the grideplate 21 I correspondingly enlarge the'activeelectric surface thereof and, at the Sametime, -I increase thereby theair damping and besides that I thereby remove any4 natural or ownvibrations of the diaphragm A20 which, as regards its mass, ist-rifiing. As in natural vibrations the maximum amplitudes pre-.dominantly occur in the central 'zone of a disc-shaped body, I enlargethe central rings or segmental webs 211 more or less in an adequatemanner and I thereby ensure. a

damping effect gradually'increasing towards the centre of thegrid-plate. Enlarged central rings 211 are not illustrated in thedrawings as the idea is readily conceivable Without illustration.

In order to adapt the arrangement so as to conform to the stressesprevailing at a time,

within narrow limits, that is to say.V to be able to change or vary thespacing, I'provide as hereinbefore described a grid-plate surface whichis bent out or permits of being bent out. .The latter modificationaffords the great advantage of ensuring thereby the like spacing at allpoints which is essential in respect of the required parallel movabilityand the free Vibratability of the diaphragm.

It has been suggested heretofore, to control and -regulate the. spacingby adjusting the diaphragm with relation to thev gridplate by means of aset-screw provided inthe centre ofthe same. But in this way itisimpossible to have the diaphragm vibrate fully and freely inasmuch asthe outer or peripheral zones thereof only will vibrate and the centralzone will remain immovable so'that a clear reproduction of the slowsound frequencies having large amplitudes cannot be brought about in asatisfactory manner.

As hereinbefore stated, I prefer to manufacture the grid-plate by way ofa pressing, casting, centrifugal casting or similar process wherebystresses in the material which are objectionable in'respect to therequired capability of the grid-plate of bending in a thoroughly uniformmanner, are avoided. The requisite perforations or apertures 21,0 arepreferably given the shape of a truncated wedge or cone, taken or viewedin section, and the solid segment-al strips or webs 211 formin gl theskeleton are similarly shaped, but inversely of course, or in otherWords. they have a wider base on the one side and a narrower top 'on theother side of the s grid-plate. It also goes without saying, that thegrid-plate must be supported in the apparatus in a manner thatdisturbing factors or momenta cannot accede from outside to -exert anyderanging action on the evenness of the eiiicient sur ace. For evenlocal variations of a few thousandths of a millimeter would reduce thedistance between the diaphragm and the grid-plate 21 to an appreciabledegree at the affected point or place and the diaphragm, therefore,would be excessively energized at such a place or places and caused tovibrate' unsymmetrii cally in the upper tones.

My invention is also applicable in the construction of microphones. Insuch a case, however, the constituent parts of the device must beconformingly simplified and reduced, respecting the dimensions thereof,as shown in Figure 13, and a somewhat greater air damping must beprovided therein. If these Y requirements are complied with a microphonetional form and size and constructed and arranged'in accordance with thetenets of electro-physics and machine construction.

I deem it also unnecessary to reiterate the operation of a loud-speakeror similar apparatus constructed according to myinvention,

for the operation. of the essential parts hasV been described in detail.Nor does it seem necessary to burden this specificationv with anexposition of the many advantages which the invention possesses, forthey will be apparent to those skilled in the `art to which thisinvention pertains.

vWhat I claim is zl. An electrostatic telephone comprising a rigid platehaving a plurality of apertures and composed, at least partially, of asuitable metal, and a vibratile diaphragm mounted in spaced relationshipand substantially parallel to the said rigid plate, the said diaphragmconsisting of a lightmetal alloy preferably aluminium alloyed with notmore than 3.5 per cent of magnesium and having a tensile strength ofmore than 20 kg. per square millimeter and the said diaphragm furtherhaving a thickness of approximately .010 to .070 millimeters and adiameter of more than Q0 centimeters and being wholly supportedperipherally and maintained highly tensioned in radial direction.

2. An electrostatic telephone comprising a rigid plate having aplurality of apertures and composed of an insulating materialsupercially coated with a metallic layer, and a vibratile diaphragmdisposed in spaced relation and substantially parallel to the said rigidplate,` the saiddiaphragmconsisting of a foil of light metal of minimumthickness of .010 to .O millimeters, and of high tensile gtrength of atleast 20 kg. per square millimeter and being wholly supportedperipherally and maintained highly tensioned in radial directions. 1 A

3. An electrostatic telephone comprising a plurality of concentric ringsinterconnected by radial webs or ribs and forming a rigid plate, and a`diaphragm mounted in substantially parallel relation to the said plate,the said diaphragm consisting of a foil of light metal of a minimumthickness and of high tensile strength, and which while being whollysupported peripherally is maintained highly tensioned in radialdirection.

4. An electrostatic telephone comprising a rigid perforated plate, asupporting ring pe-A ripherally attached to said plate by means' of aplurality of resilient pins symmetrically distributed over the peripheryof the said ring and extending substantially in radial direction, and adiaphragm adapted to freely vibrate over the entire surface, the saiddiaphragm consisting of' a light metal foil of minimum thickness andhigh tensile strength peripherally connected to the said supporting ringin order to be maintained in highly tensioned condition and insubstantially parallel spaced relationship to the said rigid plate, andan insulating layer provided between the said supporting ring and thesaid metallic diaphragm.

5. In combination with an electrostatic telephone comprising a rigidplate anda radially tensioned vibratile diaphragm disposed in spaced andparallel relationship to the said-plate, a stretching device adapted toimpart to the said diaphragm the required high tension prior toeffecting the unction thereof with the said rigid plate by means of acommon supporting ring, the said stretching device comprising a pair ofclamping rings of preferably rectangular'cross-section and of an innerdiameter somewhat greater than the outer diameter of the said ringsupporting the rigid plate and the diaphragm, the said clamping rings.being adapted to support and hold the diaphragm between them by meansof clamping screws adapted to pass through the said rings and thediaphragm interposed therebetween, 4annular grooves being provided inthe adjacent inner surfaces of the said clamping rings for therecegntion of a ring of preferablyT triangular cross-section, While inone of the said two clamping rings a plurality of axially extendingcompression screws are symmetrically distributed over the peripherythereof and adapted tobe forced, when tightened, against the said ringof preferably triangular cross-section and to thereby impart a highdegree of radial tension to the diaphragm fixed between the said twoclamping rings.

6. ln combination with an electrostatic telephone comprising a rigidplate, a radially tensioned vibratory diaphragm mounted in spaced andparallel relationship to the said plate, and a ring adapted to supportboth the said plate and the diaphragm, a method of fixing the diaphragmin tensioned condition to the said supporting ring, which comprises inimparting to the said diaphragm a radial tension preferably with the aidof a stretching device, placing the said stretching device with thediaphragm kept therein in tensioned condition over or onto the saidsupporting ring and the rigid plate disposed therein, and fixing thediaphragm to the rigid plate with the aid of a clamping ring insulated,with relationto the metallic diaphragm, by means of an insulating layer,and screws guided in insulating bushings and adapted to pass through theclamping ring; the diaphragm, the supporting ring and a second clampingring co-operating with the former, whereupon the edge portions of thediaphragm projecting are severed beyond the periphery of the supportingring.

7 An electrostatic telephone comprising a perforated, rigid platecomposed, at least partially, of metal and a vibratory diaphragmdisposed in spaced and substantially parallel relationship to the saidplate and consisting of a/light metal foil of a minimum thicknessand'high tensional strength, a supporting ring adapted to interconnectthe said diaphragm while in tensioned condition with the said rigidplate, and an insulating layer interposed between sthe latter and thediasignature at Berlin, Germany, this 24th day l phragm and possessing aslight electrical conductivity.

8. An electrostatic telephone comprising a rigid plate and avibratory-'diaphragm electrically insulated with relation 'to the saidplate andv both supported by a common ring, the said plate consisting ofa lurality of concentric rings interconnected y long and short webs orribs, a central hub and a threaded axial bore for the reception ofscrews supported b a bushing rigidly connected, y means o commonsupporting ring of the said ri 'd plate and the diaphragm, the saidscrews eing adapted to slightly bend out, when turned or tightened fromthe said rigid plate for the purpose of adjusting the distance betweenthe centers of the said plate and diaphragm.

9. An electrostatic telephone comprising a rigid plate, a vibratorydiaphragm, both mounted in a Vcommon supporting ring so as to beinsulated with relation to eachother,

and a protective grid or the like on either side of the said supportingring for reventf ing the plate and the diaphragm rom the deleteriousei'ect of mechanical and temperature influences, the saidprotectinggrids consisting preferably of perforated metal sheets bulgedtowards the outside and covered with a silk fabric or the like on theinner side and attached to the said supporting ring.

In `testimony whereof, I have affixed my o'f August, 1927.

HANS VOGT.

a supporting member, with the-

